Injury and degeneration of cartilage tissue is a major clinical challenge for several reasons. In the epidemiological sense, arthritis and other degenerative joint diseases afflict a large proportion of aging populations, which are growing at high rates in most developed nations. In the clinical sense, healing of cartilage tissue is compromised by a lack of direct blood supply. When cartilage tissue alone is damaged, i.e., in the case of a chondral lesion, local chondrocytes can only achieve limited repair. A full-thickness articular cartilage injury, or osteochondral lesion, will elicit a complete inflammatory response, but results in poor tissue reformation. As a result, a surgical approach to repair and prevention of further injury can be the only viable option. Total artificial joints have been developed and used as replacements for many years with reasonable success. Total joint replacement is nevertheless costly, invasive, carries certain risks such as blood clots, blood loss and infection, and may not provide complete restoration of function. Additionally, although significant advances have been made over the last few decades in designing robust artificial joints, they do wear out. Total joint replacement in patients younger than about 60 must be carefully considered, given the risk of the artificial joint wearing out.
Tissue engineering provides an alternative approach to joint repair. Engineered tissue, including cartilage tissue, can now be prepared in vitro and then implanted in an afflicted joint to replace damaged cartilage. The technical challenge has been how to engineer a tissue that has the biomechanical properties native to cartilage, and is also biocompatible. Various approaches have been tried with differing levels of success. One approach is to obtain cells from an acceptable donor source, and seed the cells onto some sort of scaffold that provides needed mechanical support, and then maintain the arrangement in culture with appropriate nutrients and growth factors with the expectation that the seeded cells will mature, or differentiate and mature, to the desired chondrocyte phenotype. While this approach generally holds promise, multiple technical obstacles remain, arising primarily from the difficulty in finding a suitably strong biocompatible material that also promotes chondrocyte differentiation, proliferation, phenotype retention and ability of chondrocytes to produce appropriate levels of cartilage-specific glycosoaminoglycans. Certain naturally-occurring and synthetic biopolymers have been investigated for such applications, with varying degrees of success.
While significant progress has been made in successfully engineering small amounts of certain types of cartilage, many substantial barriers remain. In particular, engineered cartilage tissue that is sufficiently robust to apply to weight-bearing joints, rather than merely to cosmetic applications, remains a continuing objective. For true functionality within a joint, the resulting tissue must demonstrate the cellular characteristics and architecture of native cartilage, while commercial viability requires that the tissue be readily generated from the relatively small amounts of source tissue that is reasonably available. The field therefore continues to search for improved methods for promoting and sustaining cartilage tissue expansion from small initial amounts of donor tissue.